In existing communication systems (legacy systems) such as an 802.16e system, a communication frame has a specified duration such as 5 ms. In the case of a system based on time division duplexing (TDD), the legacy communication frame (or simply “legacy frame”) is structured to be divided in time into a DL (downlink) portion and a UL (uplink) as illustrated in FIG. 1. In the legacy frame, guard periods TTG and RTG between the DL and UL portions serve as transition gaps to allow a user equipment of the wireless network to transition between receiving/transmitting signals (data and control) from/to a base station.
The structure of the legacy frame starts with a preamble which serves as a synchronization point for the communication between the base station and the user equipment. Also in the preamble, a cell identity is provided. That is, the cell identity within the preamble associates the legacy frame to a base station and sector.
The preamble is followed by a DL-MAP which identifies a receiving schedule for the user equipments being served by the base station. For a particular user equipment, the DL-MAP specifies which of the downlink resources, i.e., DL bursts, has been scheduled for the user equipment. The legacy frame structure also includes a UL-MAP which specifies a sending schedule for the user equipment. That is, the UL-MAP specifies which of the UL bursts has been scheduled so that the user equipment can send signals to the base station. The UL-MAP is typically provided to the user equipments in the downlink resource DL burst #1.
During operation, when the frame adhering to the legacy frame structure is received, the user equipment identifies the particular downlink and uplink resources scheduled for it and uses the identified resources and ignores the remaining resources of the frame. For example, if the DL-MAP indicates that DL burst #2 and UL burst #3 are scheduled for the user equipment, then the user equipment will listen for messages from the base station on the DL burst #2 and send messages on the UL burst #3. All other DL bursts and UL bursts are ignored by the user equipment.
While the duration of the legacy frame is fixed to 5 ms, the ratio of DL/UL portions is configurable. In FIG. 1, this means that the position of the guard period TTG is not fixed within the legacy frame. If the TTG is moved to the right, then a greater portion of the legacy frame is devoted to downlink transmissions, i.e., from the base station to the user equipments. The system is typically configured in this way if the DL traffic is expected to be larger than the UL traffic. Conversely, if the TTG is moved to the left, then a greater portion of the legacy frame is devoted to uplink transmissions. In principle the DL:UL asymmetry can be changed dynamically, but in practice the system is typically configured with a fixed DL:UL asymmetry which is the same in all cells, to avoid interference problems.
It is desirable to reduce latency both for data and control signaling. Reduced latency is important in it's own right for services that are sensitive to latency. Reduced latency, e.g., for reporting of channel measurements, can also improve system capacity and user throughput. One way to reduce the latency is to introduce shorter mini-frames inside the structure of the legacy frame as illustrated in FIG. 2. In this figure, a structure of a low-latency 802.16m frame is illustrated. The duration of the low-latency frame is the same as the legacy frame, i.e., 5 ms. However, within the low-latency frame, there are two mini-frames. As an example, each mini-frame of the low-latency frame structure can have equal duration of 2.5 ms. The first mini-frame contains a downlink portion DL-1 followed by an uplink portion UL-1. Similarly, the second mini-frame contains a downlink portion DL-2 followed by an uplink portion UL-2. Thus, in FIG. 2, the sequence of the portions within the structure of the low-latency frame is DL-1, UL-1, DL-2, and finally UL-2.
At least two problems are identified. First is the problem of a coexistence of the low-latency system with the legacy system. In FIG. 3, simplified views of the structures of the low-latency and the legacy frames are illustrated. As seen, the UL-1 portion of the low-latency frame overlaps with a part of the DL portion of the legacy frame. Similarly, the DL-2 portion of the low-latency frame overlaps with a part of the UL portion of the legacy frame. This means that if the low-latency base station is geographically co-located or located adjacent to the legacy base station, there can be simultaneous uplink and downlink transmissions. This can cause unwanted interferences.
One way to mitigate this interference problem is to simply reconfigure the legacy base station to introduce a blank period in the structure of the legacy frame to prevent simultaneous DL and UL transmissions. As illustrated in FIG. 4, the legacy base station can be designed to send legacy frames with blank periods that coincide with the UL-1 and DL-2 portions of the low-latency frames sent by the low-latency base station. During the blank period, neither the DL nor the UL resources are allocated by the legacy base station.
To the legacy user equipment (or terminal), since no resources are scheduled for itself in the blank period, the blank period appears merely as a part of the DL portion and UL portion that are scheduled for other user equipments, and thus are ignored. While the blank period prevents interferences, it does so at the cost of wasting valuable radio resources from being used in the legacy system.
The second problem is related to enabling backwards compatibility support of legacy user equipments with a low-latency base station. As illustrated in FIG. 5, this can be addressed by scheduling the resources for the legacy user equipments only in the DL-1 and UL-2 portions of the low-latency frame. That is, the scheduler in the low-latency base station will not schedule the resources in the portions UL-1 and DL-2 for communications with the legacy user equipments (as indicated by the diagonal hashing). Again from the perspective of the legacy user equipment, the UL-1 and DL-2 portions are simply treated as DL and UL resources scheduled for other user equipments and are ignored.
However, when the low-latency base station is initially installed, it is likely that a great majority of the user equipments it serves will be legacy based and very few will be low-latency user equipments. This again means that valuable radio resources will not be fully utilized.